A large scale ambient vibration survey in the area demaged by May-June 2012 seismic sequence in Emilia Romagna, Italy

Results of an extensive ambient vibration surveys carried out by different research teams in the area damaged by May–June 2012 seismic sequence in Emilia Romagna (Northern Italy) are summarized and analysed. In particular, ambient vibrations were acquired by both single station and seismic array configurations. Average horizontal to vertical spectral ratios (HVSR) at single station measurements were considered to evaluate local resonance phenomena. Despite the fact that general trends that can be detected are the effect of the subsoil configuration, H/V spectral ratios show a significant dependence on meteo-climatic conditions: in particular, HVSR amplitudes in the low frequency range (.5 Hz) correlate significantly with the sea wave activity in the Central Mediterranean. Anyway, resonance frequencies estimated from HVSR peaks appear persistent and have been used to estimate the local depth of impedance contrasts responsible for seismic resonance phenomena. To this purpose, average VS values up to any depth h(VSh) were assessed in the form of a standard power law constrained by Rayleigh dispersion curves deduced from the seismic arrays, and borehole data. In the whole area (except in the Mirandola area) two significant interfaces have been identified corresponding to two main resonance frequencies around 0.8–0.9 and 0.25–0.3 Hz respectively. The first one is attributed to a seismic impedance contrast located in the depth range 60–110 m. The second one corresponds to a deeper interface, possibly located at several hundreds of meters of depth. As concerns the Mirandola area, a single interface has been identified with depths varying in the range 50–115 m corresponding to resonant frequencies in the range 0.8–1.4 Hz. Finally a tentative geological interpretation of the above resonant interfaces is presented.